This invention relates to a pump for pressurizing and pouring molten metal into a metal mould of a hot chamber type die casting machine and more particularly to an improved construction of the pump and a goose neck interconnecting the pump and the mould.
In a hot chamber type die casting machine a container for containing molten metal is installed adjacent the die casting machine, the lower end of an injection pump is immersed in the molten metal in the container and the molten metal is injected into the mould by lowering a plunger of the pump.
The hot chamber type die casting machines have been used extensively for die casting metals having relatively low melting points such as zinc, tin and alloys thereof because of their compact construction, ready handling and capability of producing products, having high and uniform quality. However, molten metals having relatively high melting points such as aluminum and alloys thereof corrode almost all metals, and even ceramics, owing to their high activity. Such corrosion not only damage or wear the important elements of the pump but also the components of such elements dissolve in the molten metal and contaminate the composition of the cast products which results in a defect in the cast products called hard spots.
To eliminate this difficulty it has been proposed to line the inner surface of the pump cylinder with a bushing made of ceramic which is resistant to wear and not corrosed by molten metal of aluminum and alloys thereof.
Since the goose neck comprises a tube having curved opposite ends it is impossible to apply a ceramic lining on the inner surface of the goose neck after casting the main body of the pump thereabout. As the molten metal is forced to flow through the goose neck at a high speed the corrosion of the passage by the molten metal is severe with the result that the diameter of the passage increases. For this reason, the life of the goose neck is short. Since the pump and the goose neck are the largest among various component elements of a die casting machine and expensive, their lives are the important factors that determine the life of the pump so that various measures have been proposed for increasing their lives.
According to one proposal the molten metal passage of the goose neck is formed by surrounding a curved tube made of high density ceramic with cast iron which constitute the main body of the pump. With this construction the cast-in ceramic tube which acts as a core is subjected to a strong compressive force when the cast iron cools and contracts. For this reason, it is said that the ceramic tube is reenforced to withstand rupture during use. Actually, however, most of the ceramic tubes are ruptured at the time of casting due to the thermal stress created during the casting and the compressive force created at the time of cooling. Even when the ceramic tubes were not ruptured during casting, during the operation of the die casting machine they are contacted by high temperature molten metal which is at about 600.degree. C in the case of aluminum or its alloy so that the ceramic tube and the main body of the pump made of cast iron undergo thermal expansion. However, due to the difference in the thermal expansion coefficients, a gap will be formed between the ceramic tube and the main body of the pump so that the ceramic tube would be ruptured due to a high internal pressure of 200 to 300 kg/cm.sup.2 which is created at the time of injecting the molten metal. Although it has been proposed to preheat the ceramic tube for the purpose of alleviating the heat shock occurring at the time of casting, it is impossible to prevent rupture of the ceramic tube due to a strong compressive force caused by the difference in the thermal expansion coefficients of the ceramic and cast iron in the same manner as a schrincage fit. Although it is possible to alleviate the heat shock and the compressive force when heat insulating material is wrapped about the ceramic tube, during the operation of the die casting machine thermal expansion occurs with the result that a gap is created between the ceramic tube and the cast iron due to the difference in the thermal expansion coefficients so that the ceramic tube would be ruptured due to the high internal pressure of the molten metal.